Method of arranging reference signals and wireless communication base station apparatus

ABSTRACT

Disclosed is a wireless communication base station apparatus whereby it is possible to prevent degradation of throughput of LTE terminals, even when LTE terminals and LTE+ terminals are present together. In this apparatus, a setting section ( 105 ) sets in each subframe a resource block in which is arranged a reference signal that is employed solely by LTE+ terminals, based on the pattern of arrangement of reference signals employed solely by LTE+ terminals. In the case of symbols that are mapped to antennas ( 110 - 1 ) to ( 110 - 4 ), an arrangement section ( 106 ) arranges the characteristic cell reference signals employed by both LTE terminals and LTE+ terminals in all of the resource blocks in a single frame. In contrast, in the case of the symbols that are mapped to the antennas ( 110 - 5 ) to ( 110 - 8 ), the arrangement section ( 106 ) arranges in some of the resource blocks, that are set in accordance with the setting results input from a setting section ( 105 ), the characteristic cell reference signals that are employed solely by the LTE+ terminals.

TECHNICAL FIELD

The present invention relates to a reference signal allocation methodand a radio communication base station apparatus.

BACKGROUND ART

3GPP-LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) asa down link communication method. With 3GPP-LTE, a radio communicationbase station apparatus (hereinafter “base station”) transmits RSs(reference signals) using predetermined communication resources, andradio communication terminal apparatuses (hereinafter “terminals”)perform channel estimation using received reference signals todemodulate data (see Non-Patent literature 1.) In addition, terminalsuse the reference signals to perform adaptive MCS (modulation andchannel coding scheme) control, PMI (precoding matrix indicator) controlin MIMO (multiple-input multiple-output) transmission, or receivedquality measurement for adaptive scheduling. Then, terminals feedobtained PMIs and received quality information (CQI: channel qualityindicator) back to a base station.

In addition, when a base station has a plurality of antennas, the basestation enables diversity transmission. For example, a base stationtransmits a plurality of data streams from a plurality of antennas (MIMOtransmission) to allow high-speed transmission. In order to receivediversity-transmitted signals as described above with no error,terminals need to know the channel states from a group of antennas usedfor transmission in a base station, to the terminals. Therefore, RSsneed to be transmitted without interfering with each other, from allantennas provided in a base station. To realize this, 3GPP-LTE adopts amethod of transmitting RSs from respective antennas in a base station,using timings and carrier frequencies varying in the time domain and thefrequency domain.

FIG. 1 shows a configuration of a base station having four antennas (4Txbase station) anticipated with 3GPP-LTE, and FIG. 2 shows an RStransmission method in a 4Tx base station (see Non-Patent Literature 2.)Here, in FIG. 2, the vertical axis (frequency domain) is indicated by aunit of subcarriers and the horizontal axis (time domain) is indicatedby a unit of OFDM symbols. In addition, R 0, R 1, R 2 and R 3 indicateRSs transmitted from antennas 0, 1, 2 and 3 (the first, second, thirdand fourth antennas), respectively. Moreover, in FIG. 2, one block unitenclosed by a heavy-line frame (six subcarriers in the frequency domainand fourteen OFDM symbols in the time domain) is referred to as aresource block (RB.) Although one RB is composed of twelve subcarriersin 3GPP-LTE, the number of subcarriers constituting one RB is six herefor ease of explanation. In addition, each unit of one subcarrier withone OFDM symbol constituting one RB is referred to as a resource element(RE.) As seen from FIG. 2, in order to minimize RS transmissionoverhead, a 4Tx base station reduces transmission frequencies of RSs (R2and R3) from antenna 2 (third antenna) and antenna 3 (fourth antenna.)

Here, RSs shown in FIG. 2 are common to all terminals in the cellcovered by a base station, and referred to as cell-specific referencesignals. In addition, a base station may additionally transmit RSsmultiplied by a specific weight on a per terminal basis (UE-specificreference signals) for beamforming transmission.

As described above, with 3GPP-LTE, the maximum number of antennas in abase station is four, and terminals supporting 3GPP-LTE perform datademodulation and downlink signal quality measurement, using RSs (R0 toR3 shown in FIG. 2) transmitted from a base station having maximum fourantennas (4Tx base station.)

By contrast with this, LTE-advanced, which is improved 3GPP-LTE, isstudying a base station having maximum eight antennas (8Tx basestation.) Here, LTE-advanced needs to provide a base station complyingwith 3GPP-LTE in order to allow communication of terminals supportingonly base stations (4Tx base stations) in 3GPP-LTE. In other words,LTE-advanced is required to accommodate both terminals supporting only a4Tx base station (hereinafter “LTE terminals”) and terminals supportingalso an 8Tx base station (hereinafter “LTE+ terminals” or “LTE-advancedterminals”).

Citation List Non-Patent Literature

-   [NPL 1] 3GPP TS 36.213 V8.2.0-   (ftp://ftp.3gpp.org/specs/2008-03/Rel-8/36_series/36213-820.zip)-   [NPL 2] 3GPP TS 36.211 V8.2.0-   (ftp://ftp.3gpp.org/spees/2008-03/Rel-8/36_series/36211-820.zip)

SUMMARY OF INVENTION Technical Problem

With LTE-advanced, a base station needs to transmit RSs for eightantennas in order to allow LTE+ terminals to receivediversity-transmitted signals with no error. For example, as shown inFIG. 3, it may be possible to allocate R0 to R7 corresponding to eightantennas, to all RBs. By this means, LTE+ terminals are able to receivesignals with no error. Moreover, terminals can obtain the CQI and PMIfor each antenna, on a per subframe basis, so that it is possible toimprove throughput by MIMO transmission.

However, LTE terminals only know the positions of allocating RSs (R0 toR3) shown in FIG. 2. That is, LTE terminals do not know the presence ofRSs used only in LTE+ terminals, that is, R4 to R7 shown in FIG. 3.Therefore, when RSs (R4 to R7) used only in LTE+ terminals are allocatedto REs, LTE terminals recognizes the RSs as data signals and receivesthem. As described above, when LTE terminals and LTE+ terminals existtogether, the LTE terminals may not correctly receive signals. Thiscauses error rate performances and throughput of LTE terminals todeteriorate.

It is therefore an object of the present invention to provide areference signal allocation method and a radio communication basestation apparatus that makes it possible to prevent the throughput ofLTE terminals from deteriorating even if LTE terminals and LTE+terminals exist together.

Solution to Problem

The reference signal allocation method according to the presentinvention includes: allocating first reference signals to all resourceblocks in one frame, the first reference signals being used in firstradio communication terminal apparatuses supporting a radiocommunication base station apparatus having N antennas and also used insecond radio communication terminal apparatuses supporting a radiocommunication base station apparatus having more antennas than the Nantennas; and allocating second reference signals used only in thesecond radio communication terminal apparatuses to part of the resourceblocks in the one frame.

The radio communication base station apparatus according to the presentinvention that transmits first reference signals and second referencesignals, the first reference signals being used in first radiocommunication terminals supporting another radio communication basestation apparatus having N antennas and also used in second radiocommunication terminal apparatuses supporting the radio communicationbase station apparatus having more antennas than the N antennas, and thesecond reference signals being used only in the second radiocommunication terminal apparatuses adopts a configuration to include: asetting section that sets resource blocks to allocate the secondreference signals on a per subframe basis, based on allocation patternsof the second reference signals; and an allocation section thatallocates the first reference signals to all the resource blocks in oneframe and allocates the second reference signals to part of the resourceblocks set in the one frame.

Advantageous Effects of Invention

According to the present invention, it is possible to prevent throughputof LTE terminals from deteriorating even if LTE terminals and LTE+terminals exist together.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of a conventional 4Txbase station;

FIG. 2 is a drawing showing an RS transmission method in a conventional4Tx base station;

FIG. 3 is a drawing showing an RS transmission method in a conventional8Tx base station;

FIG. 4 is a block diagram showing a configuration of a base stationaccording to Embodiment 1 of the present invention;

FIG. 5 is a block diagram showing a configuration of an LTE+ terminalaccording to Embodiment 1 of the present invention;

FIG. 6 is a drawing showing an RB in which only RSs used in both LTEterminals and LTE+ terminals according to Embodiment 1 of the presentinvention;

FIG. 7 is a drawing showing an RB in which RSs used only in LTE+terminals according to Embodiment 1 of the present invention;

FIG. 8 is a drawing showing an RS allocation pattern according toEmbodiment 1 of the present invention (allocation method 1);

FIG. 9 is a drawing showing an RS allocation pattern according toEmbodiment 1 of the present invention (allocation method 1);

FIG. 10 is a drawing showing an RS allocation pattern according toEmbodiment 1 of the present invention (allocation method 1);

FIG. 11 is a drawing showing an RS allocation pattern according toEmbodiment 1 of the present invention (allocation method 2);

FIG. 12 is a drawing showing an RS allocation pattern according toEmbodiment 1 of the present invention (allocation method 2); and

FIG. 13 is a drawing showing an RS allocation pattern according toEmbodiment 1 of the present invention (allocation method 3.)

DESCRIPTION OF EMBODIMENTS

Now, embodiments of the present invention will be described in detailwith reference to the accompanying drawings. In the followingdescriptions, a base station has eight antennas and transmitstransmission data to LTE terminals and LTE+ terminals. In addition, oneframe is divided into a plurality of subframes. Moreover, a plurality ofsubcarriers in one subframe are divided into a plurality of RBs. Thatis, one RB is composed of part of subcarriers in one subframe.

Embodiment 1

FIG. 4 shows the configuration of base station 100 according to thepresent embodiment.

In base station 100, coding and modulation section 101 has N codingsections 11 and N modulation sections 12 for transmission data, andwhere N is the number of terminals to allow communication with basestation 100. In coding and modulation section 101, coding sections 11-1to 11-N perform coding processing on transmission data to terminals 1-N,and modulation sections 12-1 to 12-N perform modulation processing onencoded transmission data to generate data symbols. Here, coding andmodulation section 101 determines a coding rate and modulation scheme(i.e. MCS) in coding section 11 and modulation section 12, respectively,based on CQI information inputted from decoding sections 118-1 to 118-N.

In coding and modulation section 102, coding section 13 performs codingprocessing on information indicating a cell-specific RS allocationpattern (RS allocation information) used only in LTE+ terminals, andmodulation section 14 performs modulation processing on encoded RSallocation information to generate RS allocation information symbols.Here, base station 100 may broadcast RS allocation information to allLTE+ terminals in the cell covered by base station 100, using BCH(broadcast channel) signals.

Assigning section 103 assigns data symbols and RS allocation informationsymbols to subcarriers constituting an OFDM symbol, according to CQIinformation inputted from decoding section 118-1 to 118-N, and outputsthe result to mapping section 104.

Mapping section 104 maps symbols inputted from assigning section 103 toantennas 110-1 to 110-8, respectively. In addition, mapping section 104selects a precoding vector used in each antenna, based on PMIinformation inputted from decoding sections 118-1 to 118-N. Then,mapping section 104 multiplies the symbol mapped to each antenna by theselected precoding vector. Then, mapping section 104 outputs the symbolmapped to each antenna to allocation section 106.

Setting section 105 sets, for each subframe, cell-specific RSs (R4 toR7) transmitted from antennas 110-5 to 110-8, respectively, based on RSallocation information. To be more specific, setting section 105 setsRBs to allocate cell-specific RSs for each of a plurality of subframes,based on an allocation pattern indicating the positions to allocatecell-specific RSs (R4 to R7) used only in LTE+ terminals. Here, in theallocation pattern used in setting section 105, the cell-specific RSs(R0 to R3) used in both LTE terminals and LTE+ terminals are allocatedto all RBs in one frame, and the cell-specific RSs (R4 to R7) areallocated to part of RBs in one frame. Then, setting section 105 outputsthe setting result to allocation section 106.

Allocation section 106 adds cell-specific RSs (R0 to R7) to symbolsinputted from mapped section 104 and mapped to respective antennas. Tobe more specific, allocation section 106 allocates cell-specific RSs (R0to R3) used in both LTE terminals and LTE+terminals, to all RBs in oneframe, in symbols mapped to antennas 110-1 to 110-4. On the other hand,allocation section 106 allocates cell-specific RSs (R4 to R7) used onlyin LTE+ terminals to part of RB having been set, in symbols mapped toantennas 110-5 to 110-8, based on the setting result inputted fromsetting section 105. In addition, when transmission data directed toLTE+ terminals is assigned to an RB other than the RBs indicated by thesetting result inputted from setting section 105, allocation section 106allocates terminal specific RSs to the RB. For example, allocationsection 106 uses R4 to R7 as terminal specific RSs. Here, allocationsection 106 may use R4 to R7 multiplied by a terminal specific weight.Then, allocation section 106 outputs symbol sequences after RSallocation to IFFT (inverse fast Fourier transform) sections 107-1 to107-8.

IFFT sections 107-1 to 107-8, CP (cyclic prefix) adding sections 108-1to 108-8 and radio transmitting sections 109-1 to 109-8 are providedcorresponding to antennas 110-1 to 110-8, respectively.

IFFT sections 107-1 to 107-8 each perform IFFT on a plurality ofsubcarriers constituting an RB, to which symbols are assigned, togenerate an OFDM symbol, which is a multicarrier signal. Then. IFFTsections 107-1 to 107-8 output generated OFDM symbols to CP addingsections 108-1 to 108-8, respectively.

CP adding sections 108-1 to 108-8 each add the same signal as the endpart of an OFDM symbol to the beginning of the OFDM symbol as a CP.

Radio transmitting sections 109-1 to 109-8 perform transmissionprocessing, including D/A conversion, amplification, up-conversion andso forth, on OFDM symbols with CPs, and transmit the result torespective terminals via antennas 110-1 to 110-8. That is, base station100 transmits a plurality of data streams from antennas 110-1 to 110-8.

Meanwhile, radio receiving section 111 receives N signals transmittedfrom maximum N terminals via antennas 110-1 to 110-8, and performsreception processing, including down-conversion, A/D conversion and soforth, on these signals.

CP removing section 112 removes the CPs from signals after receptionprocessing.

FFT (fast Fourier transform) section 113 performs FFT on signals withoutCPs to obtain a signal multiplexed in the frequency domain for eachterminal, Here, a signal for each terminal includes a data signal andcontrol information containing CQI information and PMI information foreach terminal.

Demultiplexing section 114 demultiplexes a signal from each terminalinputted from FFT section 113 into a data signal and control informationfor each terminal. Then, demultiplexing section 114 outputs data signalsfrom terminals 1 to N, to demodulation sections 115-1 to 115-N,respectively, and outputs control information from terminals 1 to N, todemodulation sections 117-1 to 117-N, respectively.

Base station 100 has demodulation sections 115-1 to 115-N, decodingsections 116-1 to 116-N, demodulation sections 117-1 to 117-N anddecoding sections 118-1 to 118-N, where N is the number of terminalsthat can communicate with base station 100.

Demodulation sections 115-1 to 115-N perform demodulation processing ondata signals inputted from demultiplexing section 114, and decodingsections 116-1 to 116-N perform decoding processing on data signalsafter demodulation. By this means, it is possible to obtain receiveddata per terminal.

Demodulation sections 117-1 to 117-N perform demodulation processing oncontrol information inputted from demultiplexing section 114, anddecoding sections 118-1 to 118-N perform decoding processing on controlinformation after demodulation. Then, decoding sections 118-1 to 118-Noutput CQI information and PMI information of control information tocoding and modulation section 101, assigning section 103 and mappingsection 104.

Next, terminal 200 (LTE+terminal) according to the present embodimentwill be explained. FIG. 5 shows the configuration of terminal 200according to the present embodiment.

In terminal 200 shown in FIG. 5, radio receiving sections 202-1 to 202-8CP removing sections 203-1 to 203-8, FFT sections 204-1 to 204-8 andextracting sections 205-1 to 205-8 are provided corresponding toantennas 201-1 to 201-8, respectively.

Radio receiving sections 202-1 to 202-8 receive OFDM symbols transmittedfrom base station 100 (FIG. 4) via antennas 201-1 to 201-8 and performreception processing, including down-conversion, A/D conversion and soforth, on these OFDM symbols.

CP removing sections 203-1 to 203-8 remove the CPs from the OFDM symbolsafter reception processing.

FFT sections 204-1 to 204-8 perform FFT on OFDM symbols without CPs toobtain frequency domain signals.

Extracting sections 205-1 to 205-8 extract cell-specific RSs (R0 to R7)and terminal specific RS (for example, R4 to R7 multiplied with terminalspecific weighting) from signals inputted from FFT sections 204-1 to204-8, based on RS allocation information inputted from decoding section211. Then, extracting sections 205-1 to 205-8 output cell-specific RSsto channel estimating section 206 and measuring section 212, and outputsterminal specific RSs to channel estimating section 206. In addition,extracting sections 205-1 to 205-8 output signals inputted from FFTsections 204-1 to 204-8 to spatial reception processing section 207.Here, terminal 200 may obtain RS allocation information by receiving BCHsignals containing RS allocation information from base station 100.

Channel estimating section 206 performs channel estimation usingcell-specific RSs and terminal specific RSs inputted from extractingsections 205-1 to 205-8, and outputs the channel estimation result tospatial reception processing section 207.

Spatial reception processing section 207 performs spatial demultiplexingprocessing on signals inputted from extracting sections 205-1 to 205-8,respectively, that is, signals received by antennas 201-1 to 201-8,respectively, using the channel estimation result inputted from channelestimating section 206. Then, spatial reception processing section 207outputs data signals of demultiplexed data streams to demodulationsection 208, and outputs RS allocation information of the demultiplexeddata streams to demodulation section 210.

Demodulation section 208 performs demodulation processing on datasignals inputted from spatial reception processing section 207, anddecoding section 209 performs decoding processing on data signals afterdemodulation. By this means, it is possible to obtain received data.

Demodulation section 210 performs demodulation processing on RSallocation information inputted from spatial reception processingsection 207, and decoding section 211 performs decoding processing on RSallocation information after demodulation. Then, decoding section 211outputs RS allocation information after decoding to extracting sections205-1 to 205-8.

Meanwhile, measuring section 212 performs CQI measurement for each ofantennas 201-1 to 201-8 and PMI estimation to obtain good receivedquality, using cell-specific RSs (R0 to R7) inputted from extractingsections 205-1 to 205-8. Then, measuring section 212 outputs CQIinformation indicating the measured CQI and PMI indicating the estimatedPMI to coding section 215, as control information.

Coding section 213 performs coding processing on transmission data, andmodulation section 214 performs nodulation processing on encodedtransmission data to generate a data symbol. Then, modulation section214 outputs the generated data symbol to multiplexing section 217.

Coding section 215 performs coding processing on control informationcontaining CQI information and PMI information inputted from measuringsection 212, and modulation section 216 performs modulation processingon encoded control information to generate a control information symbol.Then, modulation section 216 outputs the generated control informationsymbol to multiplexing section 217.

Multiplexing section 217 multiplexes the data symbol inputted frommodulation section 214 and the control information symbol inputted frommodulation section 216, and outputs a multiplexed signal to IFFT section218.

IFFT section 218 performs IFFT on a plurality of subcarriers to whichthe signal inputted from multiplexing section is assigned and outputs asignal after IFFT to CP adding section 219.

CP adding section 219 adds the same signal as the end part of the signalinputted from IFFT section 218 to the beginning of the signal as a CP.

Radio transmitting section 220 performs transmission processing,including D/A conversion, amplification, up-conversion and so forth, onthe signal with a CP, and transmits the result from antenna 201-1 tobase station 100 (FIG. 4.)

Next, a cell-specific RS allocation method according to the presentembodiment will be described.

In the following descriptions, as shown in FIG. 8, for example, oneframe is composed of five subframes (subframes 0 to 4.) In addition, acase will be explained as an example where a plurality of subcarriers inone subframe are evenly divided into four RBs (RB 0 to RB 3). Inaddition, as shown in FIG. 6 and FIG. 7, one RB is composed of sixsubcarriers x one subframe. In addition, cell-specific RS (R0 to R3)used in both LTE terminals and LTE+ terminals are allocated to presetREs in an RB as shown in FIG. 6 and FIG. 7. In addition, cell-specificRSs (R4 to R7) used only in LTE+ terminals are allocated to preset REsin an RB as shown in FIG. 7.

In addition, in the following descriptions, an RB (FIG. 6) to allocatefour RSs, R 0 to R 3, is represented as 4RS and an RB (FIG. 7) toallocate eight RSs, R 0 to R 7, is represented as 8RS, as shown in FIG.8. That is, in FIG. 8, while cell-specific RSs (R0 to R3) used in bothLTE terminals and LTE+ terminals are allocated to all RBs in one frame,RSs (R4 to R7) used only in LTE+ terminals are allocated to only the RBsrepresented as 8RSs.

<Allocation Method 1 (FIG. 8)>

This allocation method allocates cell-specific RSs used only in LTE+terminals, to only part of RBs in one frame.

Here, if cell-specific RSs used only in LTE+ terminals are fixedlyallocated to only limited part of frequency bands in one frame, basestation 100 can only assign data signals from both LTE+ terminals andLTE terminals to only limited frequency bands. For example, ifcell-specific RSs (R4 to R7) used only in LTE+ terminals are fixedlyallocated to RB 0 and RB 1 among RB 0 to RB 3 in subframes 0 to 4 of oneframe, base station 100 can only allocate data signals directed to LTEterminals to only RB 2 and RB 3. That is, if cell-specific RSs used onlyin LTE+ terminals are fixedly allocated to only limited part offrequency bands in one frame, RBs to which LTE terminals can be assignedare limited, so that frequency scheduling effect deteriorates.

Therefore, with this allocation method, cell-specific RSs (R4 to R7)used only in LTE+ terminals are allocated to RBs in different frequencybands between neighboring subframes.

To be more specific, as shown in FIG. 8, R4 to R7 are allocated to RB 0in subframe 0, R4 to R7 are allocated to RB 1 in subframe 1, R4 to R7are allocated to RB 2 in subframe 2, R4 to R7 are allocated to RB 3 insubframe 3 and R4 to R7 are allocated to RB 0 in subframe 4.

That is, setting section 105 (FIG. 4) in base station 100 sets RB 0 forsubframe 0 and sets RB 1 for subframe 1, as RBs to allocatecell-specific RSs (R4 to R7) used only in LTE+ terminals as shown inFIG. 8. The same applies to subframes 2 to 4.

Allocation section 106 allocates R4 to R7 to respective correspondingREs of RB 0 in subframe 0, and allocates R4 to R7 to respectivecorresponding REs of RB 1 in subframe 1 as shown in FIG. 7. The sameapplies to subframes 2 to 4.

As shown in FIG. 8, R4 to R7 are allocated to only five RBs, amongtwenty RBs (“five subframes from RB 0 to RB 4”×“four RBs from RB 0 to RB3”.) That is, only R0 to R3 that can be received by LTE terminals aretransmitted using fifteen RBs (4RSs shown in FIG. 8) other than part ofRBs (8RSs shown in FIG. 8) to allocate R4 to R7. Therefore, base station100 is able to assign LTE terminals to RBs (4RSs shown in FIG. 8) otherthan part of RBs (8RSs shown in FIG. 8) to allocate R4 to R7. By thismeans, LTE terminals do not receive REs to which R4 to R7 have beenallocated, as data symbols by mistake, so that it is possible to preventdeterioration of error rate performances.

In addition, as shown in FIG. 8, RBs (8RSs shown in FIG. 8) to allocateR4 to R7 are allocated to RBs in different frequency bands betweenneighboring subframes. To be more specific, as shown in FIG. 8, while R4to R7 are allocated to RB 0 in subframe 0, they are allocated to RB 1 insubframe 1 next to subframe 0, where RB 1 is different from RB 0 in thefrequency band. Likewise, R4 to R7 are allocated to RB 2 in subframe 2next to subframe 1 in different frequency bands from RB 1. The sameapplies to subframes 3 and 4. That is, R4 to R7 are allocated to RBsshifted one RB per subframe in the frequency domain.

By this means, terminal 200 (LTE+terminal) can perform CQI measurementand PMI estimation using eight cell-specific RSs (R0 to R7) in any oneRB in one subframe, and update CQI and PMIs for all RBs 0 to 3. Then,terminal 200 (LTE+terminal) feeds the obtained CQI and PMIs back to basestation 100. In addition, base station 100 performs adaptive MCS controlbased on the fed back CQI, and performs MIMO transmission oftransmission data using the fed back PMIs. Here, terminal 200(LTE+terminal) may feed the CQI and PMI obtained from each subframe backto the base station on a per subframe basis. This allows terminal 200(LTE+terminal) to reduce the amount of feedback per subframe and feednewer CQI and PMI, that is, more accurate CQI and PMI per RB back to thebase station. In addition, terminal 200 (LTE+terminal) may obtain allCQI and PMIs of RB 0 to RB 3 and then feed them back to the base stationat the same time.

Here, it is anticipated that high-speed transmission using eightantennas in base station 100 (MIMO transmission) is performed in amicrocell with a small cell radius. Therefore, high-speed transmissionusing eight antennas in base station 100 supports only LTE+ terminalsmoving at low speeds. Therefore, as shown in FIG. 8, even if a long timeinterval corresponding to four subframes is required to perform CQImeasurement and PMI estimation in all RBs, deterioration of the accuracyof CQI measurement and PMI estimation is low because channel qualityvariation over four subframes is moderate. That is, base station 100 isable to perform adaptive MCS control and MIMO transmission usingsufficiently accurate CQI and PMIs from terminal 200 (LTE+terminal), andtherefore improve throughput.

In addition, when data of terminal 200 (LTE+terminal) is assigned to RBs(4RSs shown in FIG. 8) not to allocate R4 to R7, base station 100allocates terminal specific RSs for data demodulation (R4 to R7multiplied by a terminal specific weight), to RBs to which data isassigned, and transmits them. That is, by using terminal specific RSs,base station 100 is able to assign data signals directed to LTE+terminals not only to RBs (8RSs shown in FIG. 8) to allocate R4 to R7but also to any of RBs 0 to 3. This allows base station 100 to throw offthe limitation of schedulers for LTE+terminal assignment, so that it ispossible to improve frequency scheduling effect.

Here, RBs used to transmit terminal specific RSs vary depending on thedecision made by base station 100 that which RBs are assigned to LTE+terminals, and each LTE+terminal is reported only the RB assigned tothat LTE+terminal from base station 100, and therefore only know thepresence of the terminal specific RSs transmitted to the RB assigned toLTE+terminal. That is, other LTE+ terminals cannot perform CQImeasurement and PMI estimation using terminal specific RSs. However,with this allocation method, a cell-specific RS is transmitted to anyone of RBs on a per subframe basis, so that it is possible to perform.CQI measurement and PMI estimation even if other LTE+ terminals do notknow terminal specific RSs.

As described above, according to this allocation method, cell-specificRSs used only in LTE-terminals are allocated to only part of RBs of aplurality of RBs in one frame. By this means, a base station can assigndata signals directed to LTE terminals to RBs other than RBs to allocatecell-specific RSs used only in LTE+ terminals. Therefore, LTE terminalsdo not receive cell-specific RSs used only in LTE-terminals as datasignals by mistake, so that it is possible to prevent deterioration oferror rate performances. Therefore, with this allocation method, it ispossible to prevent throughput of LTE terminals from deteriorating evenif LTE terminals and LTE+ terminals exist together. In addition, whendata signals directed to LTE+ terminals are assigned to RBs not toallocate cell-specific RSs used only in LTE+ terminals, a base stationallocates terminal specific RSs to RBs. By this means, a base stationcan assign data signals directed to LTE+ terminals to all RBs, so thatit is possible to improve frequency scheduling effect.

In addition, with this allocation method, cell-specific RSs used only inLTE+ terminals are allocated to RBs in different frequency bands betweenneighboring subframes, and where these RBs are shifted one RB persubframe. By this means, LTE+ terminals can reliably receivecell-specific RSs over a plurality of consecutive subframes even in RBsto which their data signals are not assigned. Therefore, LTE+ terminalsare able to accurately perform CQI measurement and PMI estimation in allfrequency bands.

Here, with this allocation method, it may be possible to use RSallocation patterns that vary in the time domain and frequency domainper cell, For example, one of two neighboring base stations may use theallocation pattern shown in FIG. 8, while the other base station may usethe allocation pattern shown in FIG. 9. R4 to R7 are allocated to RBs 0,1, 2, 3 and 0 in the order of subframes 0, 1, 2, 3 and 4 in theallocation pattern shown in FIG. 8, while R4 to R7 are allocated to RBs0, 2, 1, 3 and 0 in the order of subframes 0, 1, 2, 3 and 4 in theallocation pattern shown in FIG. 9. That is, in the allocation patternshown in FIG. 9, R4 to R7 are allocated to part of RBs in one frame,where the RBs are shifted in the frequency domain every plurality of RBs(two RBs, here) on a per subframe basis. Otherwise, one of twoneighboring base stations may use the allocation pattern shown in FIG.8, while the other base station may use the allocation pattern shown inFIG. 10. In the allocation pattern shown in FIG. 10, R4 to R7 areallocated to RBs 1, 2, 3, 0 and 1 in the order of subframes 0, 1, 2, 3and 4. That is, R4 to R7 are allocated to RBs shifted one RB at a timefrom RB 0 in subframe 0 in the allocation pattern shown in FIG. 8, whileR4 to R7 are allocated to RBs shifted one RB at a time from RB 1 insubframe 0 in the allocation pattern shown in FIG. 10. By this means, itis possible to reduce the possibility that R4 to R7 are allocated in thesame frequency band and the same time domain in a plurality of cells.Generally, cell-specific RSs are transmitted directed to all terminalsin a cell, and therefore transmitted using a higher transmission powerthan data symbols. That is, a terminal located in the cell boundaryreceives not only cell-specific RSs from the cell to which the terminalbelongs, but also cell-specific RSs from neighboring cells, so thatinter-cell interference with cell-specific RSs is increased. However, asdescribed above, it is possible to reduce inter-cell interference withcell-specific RSs by using allocation patterns varying in the timedomain and the frequency domain on a per cell basis, so that theaccuracy of CQI measurement and PMI estimation in each terminal isimproved.

In addition, according to the present invention, a configuration ispossible where one frame is composed of four subframes, and one framecorresponds to one cycle of an allocation pattern in which R4 to R7 areallocated to all RBs. In this case, an LTE+terminal moving from aneighboring cell by handover and so forth is able to receivecell-specific RSs (R4 to R7) even if it does not know frame numbers.

<Allocation Method 2 (FIG. 11)>

While cell-specific RSs used only in LTE+ terminals are allocated to oneRB in one subframe with allocation method 1, cell-specific RSs used inLTE+ terminals are allocated to a plurality of RBs in one subframe Withthis allocation method.

When a terminal moves at a low speed, channel quality variation betweena base station and the terminals is moderate. On the other hand, when aterminal moves at a high speed, channel quality variation between a basestation and the terminal is significant. That is, when a terminal movesat a higher speed, the channel quality variation per subframe issignificant. Accordingly, in a case in which a terminal moves at ahigher speed, if RSs obtained in a subframe early for a long timeinterval is used, it is not possible to correctly reflect the channelquality at this time, so that accuracy of CQI measurement and PMIestimation deteriorates.

Therefore, with this allocation method, cell-specific RSs used only inLTE+ terminals (R4 to R7) are allocated to a plurality of RBs in onesubframe,

To be more specific, as shown in FIG. 11, R4 to R7 are allocated to RB 0and RB 1 in subframe 0, R4 to R7 are allocated to RB 2 and RB 3 insubframe 1, R4 to R7 are allocated to RB 0 and RB 1 in subframe 2, R4 toR7 are allocated to RB 2 and RB 3 in subframe 3 and R4 to R7 areallocated to RB 0 and RB 1 in subframe 4.

That is, setting section 105 (FIG. 4) in base station 100 sets two RBs,RB 0 and RB 1, in subframe 0 and sets two RBs, RB 2 and RB 3, insubframe 1, as RBs to allocate cell-specific RSs (R4 to R7) used only inLTE+ terminals as shown in FIG. 11. The same applies to subframes 2 to4.

In addition, allocation section 106 allocates R4 to R7 to respectivecorresponding REs of RB 0 and RB 1 in subframe 0, and allocates R4 to R7to respective corresponding REs of RB 2 and RB 3 in subframe 1 as shownin FIG. 7. The same applies to subframes 2 to 4.

As shown in FIG. 11, R4 to R7 are allocated to ten RBs, among twenty RBsin one frame. That is, only R0 to R3, which can be received by LTEterminals, are transmitted in ten RBs (4RSs shown in FIG. 11) other thanpart of RBs (8RSs shown in FIG. 11.) to allocate R4 to R7. This allowsLTE terminals to prevent deterioration of error rate performances in thesame way as in allocation method 1 (FIG. 8.)

In addition, while terminal 200 (LTE+terminal) in allocation method 1(FIG. 8) can receive cell-specific RSs (R0 to R7) allocated to all RBsusing four subframes, terminal 200 (LTE+terminal) in FIG. 11 can receivecell-specific RSs (R0 to R7) allocated to all RBs using two subframes.In other words, while terminal 200 (LTE+terminal) in allocation method 1(FIG. 8) can receive R4 to R7 in one RB every four subframes, terminal200 (LTE+terminal) in FIG. 11 can receive R4 to R7 in one RB every twosubframes. That is, terminal 200 (LTE+terminal) in this allocationmethod can receive new R4 to R7 at narrower subframe intervals than inallocation method 1.By this means, with this allocation method, it ispossible to update channel quality in all RBs at narrower subframeintervals than in allocation method 1. Therefore, even if terminal 200(LTE+terminal) moves at a high speed, it is possible to use channelquality measured using cell-specific RSs in a subframe received at anearlier time, so that terminal 200 can improve the accuracy of CQImeasurement and PMI estimation.

Here, this allocation method may use an allocation pattern shown in FIG.12 instead of the allocation pattern shown in FIG. 11. That is,cell-specific RSs (R4 to R7) used only in LTE+ terminals may beallocated to a plurality of discrete RBs in the frequency domain in onesubframe.

To be more specific, as shown in FIG. 12, R4 to R7 are allocated to RB0, and RB 2, which does not continue to RB 0 in the frequency domain, insubframe 0, and R4 to R7 are allocated to RB 1, and RB 3, which isdiscontinued to RB 1 in the frequency domain, in subframe 1. The sameapplies to subframes 2 to 4.

As described above, by allocating cell-specific RSs used only in LTE+terminals to a plurality of discrete RBs in the frequency domain in onesubframe, RBs (4RSs shown in FIG. 12) to which data signals directed toLTE terminals can be assigned, are also discontinued in the frequencydomain in base station 100. Therefore, even if frequency selectivity ismoderate, base station 100 is able to assign RBs distributed in thefrequency domain, to LTE terminals. By this means, it is possible toprevent base station 100 from continuously assigning LIE terminals toRBs with poor received quality, so that it is possible to improvefrequency schedule effect.

Here, with this allocation method, the number of RBs to which LTEterminals can be assigned is less than in allocation method 1 (FIG. 8.)However, RBs to which LTE terminals can be allocated vary on a persubframe basis, so that base station 100 is able to assign UTE terminalsto RBs with better channel quality in one of two consecutive subframes.That is, deterioration of frequency scheduling effect due to decrease inthe number of RBs to which LTE terminals can be allocated is low.

As described above, according to this allocation method, cell-specificRSs used only in LTE+ terminals are allocated to part of a plurality ofRBs in one subframe. By this means, it is possible to produce the sameeffect as in allocation method 1. In addition, according to thisallocation method, even if there is an LTE+terminal moving at a highspeed, the LTE+terminal can perform CQI measurement and PMI estimationusing RSs received in a newer subframe, that is, RSs reflecting thechannel quality at this time.

Here, with this allocation method, base station 100 may switch betweenthe allocation pattern shown in FIG. 11 and the allocation pattern shownin FIG. 12, depending on channel states (frequency selectivity) in acell. That is, setting section 105 in base station 100 may switch thefrequency interval of a plurality of RBs to allocate R4 to R7 in onesubframe, depending on channel states in a cell. By this means, basestation 100 allows scheduling suitable for channel states, and thereforemakes it possible to increasingly improve frequency scheduling effect.

<Allocation Method 3 (FIG. 13)>

With this allocation method, cell-specific RSs used only in LTE+terminals are allocated to part of RBs at predetermined subframeintervals.

As described above, if a terminal moves at a low speed, channel qualityvariation between a base station and the terminal is moderate.Therefore, in a case in which a terminal moves at a low speed, even ifchannel quality provided by using RSs obtained in a subframe early for along time interval, is used as the channel quality at this time, theaccuracy of CQI measurement and PMI estimation does not deteriorate.Therefore, when a terminal moves at a low speed, cell-specific RSs usedonly in LTE+ terminals need not to be allocated to RBs on a per subframebasis, unlike in allocation method 1 (FIG. 8.)

Therefore, with this allocation method, cell-specific RSs (R4 to R7)used only in LTE+ terminals are allocated to part of RBs atpredetermined subframe intervals.

In the following descriptions, the predetermined subframe interval istwo subframes. In addition, like in allocation method 2 (FIG. 12),cell-specific RSs used only in LTE+ terminals (R4 to R7) are allocatedto a plurality of discrete RBs in the frequency domain in one subframe.

To be more specific, as shown in FIG. 13, R4 to R7 are allocated to RB 0and RB 2 in subframe 0, R4 to R7 are allocated to RB 1 and RB 3 insubframe 2 two subframes apart from subframe 0, and R4 to R7 areallocated to RB 0 and RB 2 in subframe 4 two subframes apart fromsubframe 2.

That is, as shown in FIG. 13, setting section 105 (FIG. 4) in basestation 100 sets two RBs, RB 0 and RB 2, in subframe 0, sets two RBs, RB1 and RB 3, in subframe 2, and sets two RBs, RB 0 and RB 2, in subframe4, as RBs to allocate cell-specific RSs (R4 to. R7) used only in LTE+terminals. Meanwhile, setting section 105 sets no RB to allocate R4 toR7 in subframe 1 and subframe 3.

In addition, allocation section 106 allocates R4 to R7 to respectivecorresponding REs of RB 0 and RB 2 in subframe 0, allocates R4 to R7 torespective corresponding REs of RB 1 and RB 3 in subframe 2, andallocates R4 to R7 to respective corresponding REs of RB 0 and RB 2 insubframe 4, as shown in FIG. 7.

As shown in FIG. 13, R4 to R7 are allocated to only six RBs of twentyRBs in one frame. That is, only R0 to R3 that can be received by LTEterminals are transmitted in fourteen RBs (4RSs shown in FIG. 13) otherthan part of RBs (8RSs shown in FIG. 13) to allocate R4 to R7.Therefore, LTE terminals are able to prevent deterioration of error rateperformances in the same way as in allocation method 1 (FIG. 8)

In addition, terminal 200 (LTE+terminal) can receive cell-specific RSs(R0 to R7) allocated to all RBs using four subframes in FIG. 13.Accordingly, terminal 200 (LTE+terminal) can update CQI and PMI for eachRB every four subframes in the same way as in allocation method 1 (FIG.8.)

As described above, according to this allocation method, cell-specificRSs used only in LTE+ terminals are allocated to part of RBs atpredetermined subframe intervals. By this means, it is possible toreduce the number of cell-specific RSs used only in LTE+ terminals inone frame while the accuracy of CQI measurement and PMI estimation inLTE+ terminals is maintained, and also possible to increase the numberof RBs to assign data signals directed to LTE terminals. Therefore,according to this allocation method, even if LTE terminals and LTE+terminals exist together, it is possible to maximally reserve RBsassigned to LTE terminals, so that it is possible to prevent throughputof LTE terminals from deteriorating in the same way as in allocationmethod 1.

Here with this allocation method, although the predetermined subframeinterval is limited to two subframes, the predetermined subframeinterval is not limited to two subframes. For example, base station 100may set predetermined subframe intervals according to the moving speedof an LTE+terminal. To be more specific, when an LTE+terminal moves at alower speed, base station 100 may set predetermined subframe intervalsgreater because channel quality variation is moderate.

Allocation methods 1 to 3 according to the present embodiment have beenexplained.

As described above, according to the present embodiment, even if LTEterminals and LTE+ terminals exist together, it is possible to preventthe throughput of LTE terminals from deteriorating. In addition,according to the present embodiment, a base station throws off thelimitation of scheduling RBs to assign LTE+ terminals, and the number ofRBs to assign LTE terminal increases, so that it is possible to performfrequency scheduling on more frequency bands.

Embodiment 2

With the present embodiment, a case will be explained where allocationmethods 1 to 3 according to Embodiment 1 are selectively employeddepending on cell environments,

As described above, while allocation method 1 makes it possible toreduce the number of RBs to allocate cell-specific RSs (R4 to R7) usedonly in LTE+ terminals as compared to allocation method 2, allocationmethod 2 allows a base station to transmit cell-specific RSs (R4 to R7)allocated to all RBs in narrower subframe intervals than in allocationmethod 1. That is, while allocation method 1 makes it possible toreserve a greater number of RBs to allocate LTE terminals in one framethan in allocation method 2, allocation method 2 allows the subframeinterval to be narrower where LTE+ terminals can update channel qualityin the entire frequency domain than in allocation method 1.

Likewise, while allocation method 3 makes it possible to reserve agreater number of RBs to allocate LTE terminals in one frame than inallocation method 2, allocation method 2 allows the subframe interval tobe narrower where LTE+ terminals can update channel quality in theentire frequency domain than in allocation method 3.

That is, the relationship between the number of RBs to which LTEterminals can be assigned in one frame and subframe intervals in whichLTE+ terminals can update channel quality in all RBs is trade-offbetween allocation method 1 (allocation method 3) and allocation method2.

Therefore, setting section 105 (FIG. 4) according to the presentembodiment sets RBs to allocate cell-specific RSs (R4 to R7) byswitching between allocation method 1 (allocation method 3) andallocation method 2 according to Embodiment 1,depending on cellenvironments.

Now, switching methods 1 and 2 in setting section 105 according to thepresent embodiment will be explained.

<Switching Method 1>

With this switching method, the allocation method is changed accordingto the number of LTE terminals in a cell.

As described above, base station 100 (FIG. 4) is able to assign LTE+terminals to RBs other than the RBs to allocate cell-specific RSs (R4 toR7) by allocating R4 to R7, which are terminal specific RSs. By contrastwith this, base station 100 can only assign LTE terminals to only RBsother than the RBs to allocate cell-specific RSs (R4 to R7.) Therefore,when the number of LTE terminals is greater, base station 100 needs toreserve more RBs to which LTE terminals can be assigned, that is, RBsother than the RBs to allocate cell-specific RSs used only in LTE+terminals. To be more specific, when the number of LTE terminals isgreater, base station 100 needs to reduce the number of RBs to allocatecell-specific RSs used only in LTE+ terminals.

On the other hand, when the number of LTE terminals is smaller, basestation 100 can reserve more RBs to allocate cell-specific RSs used onlyin LTE+ terminals. This allows terminal 200 (FIG. 5) to receivecell-specific RSs used only in LTE+ terminals in more RBs, frequencyscheduling effect in LTE+ terminals is improved.

Therefore, when the number of LTE terminals is great, setting section105 sets RBs to allocate R4 to R7 using allocation method 1 (allocationmethod 3), and, when the number of LTE terminals is small, sets RBs toallocate R4 to R7 using allocation method 2. To be more specific,setting section 105 switches between allocation methods by comparing thenumber of LTE terminals with a preset threshold. That is, when thenumber of LTE terminals is equal to or higher than the threshold,setting section 105 switches the allocation method to allocation method1 (allocation method 3), and, when the number of LTE terminals is lowerthan the threshold, switches the allocation method to allocation method2. That is, setting section 105 changes the number of cell-specific RSsused only in LTE+ terminals, depending on the number of LTE terminals ina cell.

By this means, when the number of LTE terminals is great, base station100 employs allocation method 1 (allocation method 3), and therefore isable to maximally reserve RBs to which LTE terminals can be assigned,while allocating cell-specific RSs used only in LTE+ terminals to partof RBs. On the other hand, when the number of LTE terminals is small,base station 100 employs allocation method 2, and therefore is able tomaximally reserve RBs to allocate cell-specific RSs used only in LTE+terminals, while reserving RBs to which LTE terminals can be assigned.

As described above, according to this switching method, when the numberof LTE terminals in a cell is great, a base station switches theallocation method to an allocation method to allow preferentialacquisition of RBs to which LTE terminals can be assigned. On the otherhand, when the number of LTE terminals in a cell is small, a basestation switches the allocation method to an allocation method to allowpreferential acquisition of frequency scheduling effect by narrowingsubframe intervals in which LTE+ terminals can receive cell-specific RSsin all frequency bands. By this means, whether the number of LTEterminals is great or small, it is possible to produce frequencyscheduling effect while reserving RBs to assign LTE terminals.

<Switching Method 2>

With this switching method, allocation methods are switched depending onthe moving speed of an LTE+ terminal in a cell.

As described above, when an LTE+terminal moves at a higher speed,channel quality variation is significant, so that terminal 200 needs toupdate channel quality for each RB at narrower time intervals, that is,at narrower subframe intervals, in order to perform CQI measurement andPMI estimation without deterioration of accuracy.

On the other hand, an LTE+terminal moves at a lower speed, channelquality variation is moderate, so that terminal 200 can perform CQImeasurement and PMI estimation without deterioration of the accuracyeven if the channel quality of each RB is updated at wide timeintervals, that is, at wide subframe intervals.

Therefore, when an LTE+terminal moves at a low speed, setting section105 sets RBs to allocate R4 to R7 using allocation method 1 (allocationmethod 3), and, when an LTE+terminal moves at a high speed, sets RBs toallocate R4 to R7 using allocation method 2. To be more specific,setting section 105 switches allocation methods by comparing the movingspeed of an LTE+ terminal with a preset threshold. That is, when thereare only LTE+ terminals moving at moving speeds equal to or lower thanthe threshold, setting section 105 switches the allocation method toallocation method 1 (allocation method 3), and, when there are LTE+terminals moving at moving speeds higher than the threshold, switchesthe allocation method to allocation method 2. That is, setting section105 changes intervals of subframes to allocate sell specific RSs usedonly in LTE+ terminals, depending on the moving speed of a LTE terminal.

By this means, when LTE+ terminals move at low speeds, base station 100employs allocation method 1 (allocation method 3), and therefore is ableto maximally reserve RBs to which LTE-terminals can be assigned whileminimizing RBs to allocate cell-specific RSs used only in LTE+terminals. On the other hand, when LTE+ terminals move at high speeds,base station 100 employs allocation method 2, and therefore is able tomaximally reserve RBs to allocate cell-specific RSs used only in LTE+terminals while reserving RBs to which LTE terminals can be assigned.

As described above, according to this switching method, when LTE+terminals move at low speeds in a cell, base station switches theallocation method to an allocation method to allow preferentialacquisition of RBs to which LTE terminals can be assigned. On the otherhand, when LTE+ terminals move at high speeds in a cell, a base stationswitches the allocation method to an allocation method to allowpreferential acquisition of frequency scheduling effect by narrowingsubframe intervals in which LTE+ terminals can receive cell-specific RSsin all frequency bands. By this means, whether LTE+ terminals in a cellmove at high or low speeds, it is possible to produce frequencydiversity effect in LTE+ terminals while reserving RBs to assign LTEterminals in the same way as in switching method 1.

Switching methods 1 and 2 in setting section 105 according to thepresent embodiment have been explained.

As described above, according to the present embodiment, methods ofallocating cell-specific RSs used only in LTE-terminals are switcheddepending on cell environments, so that it is possible to maximallyproduce frequency scheduling effect in LTE+ terminals while maximallyreserving RBs to which LTE terminals can be assigned, depending on cellenvironments.

Here, with the present embodiment, after switching between theallocation pattern of allocation method 1 (allocation method 3) and theallocation pattern of allocation method 2, base station 100 (FIG. 4) maybroadcast information indicating that the allocation pattern has beenswitched, to all terminals 200 (LTE+ terminals) using BCH signals. Here,allocation patterns 1 to 3 are shared between base station 100 andterminals 200. By this means, base station 100 can switch betweenallocation patterns depending on cell environments without reporting anallocation pattern to terminal 200 every time the allocation pattern isswitched. In addition, base station 100 may individually reportinformation indicating that the allocation pattern has been switched toLTE+ terminals, using RRC (radio resource control) signaling.

The embodiments according to the present invention have been described.

Here, according to the present invention, the transmission power ofcell-specific RSs (R4 to R7) used only in LTE+ terminals, amongcell-specific RSs (R0 to R7), may be lower than the transmission powerof cell-specific RSs (R0 to R3) used in both LTE terminals andLTE+terminals. It is anticipated that terminals (LTE terminals and LTE+terminals) to receive signals transmitted from a base station using fourantennas are located all over a cell. By contrast with this, it isanticipated that terminals to receive signals transmitted at a highspeed from a base station using six antennas are located near the centerof a cell where channel quality is good. Therefore, a base station canimprove efficiency of RS transmission by transmitting cell-specific RSs(R4 to R7) used only in LTE+ terminals, at lower power than the power totransmit cell-specific RSs (R0 to R3) used in both LTE terminals andLTE+ terminals. Moreover, according to the present embodiment, thenumber of RS symbols per RB (i.e. RS allocation density) ofcell-specific RSs (R4 to R7) used only in LTE+ terminals, amongcell-specific RSs (R0 to R7), may be lower than the allocation densityof cell-specific RSs (R0 to R3) used in both LTE- terminals and LTE+terminals.

In addition, with the above-described embodiments, a communicationsystem in which LTE terminals and LTE+ terminals exist together, hasbeen explained. However, the present invention is not limited to acommunication system in which LTE terminals and LTE+ terminals existtogether, and is applicable to, for example, a communication system inwhich terminals supporting only a base station having N antennas andterminals supporting a base station having more than N antennas existtogether. In addition, the present invention is applicable to a case inwhich terminal 1 and terminal 2 exist together, and where terminal 1operates in communication system A and terminal 2 operates in onlycommunication system B of an earlier version than communication system Ain which terminal 1 operates.

Moreover, with the above-described embodiments, a case has beenexplained where the number of subframes constituting one frame is five,and a plurality of subcarriers in one sub frame is divided into fourRBs. However, according to the present invention, the number ofsubframes constituting one frame is not limited to five, and also thenumber of RBs into which a plurality of subcarriers in one subframe isdivided, is not limited to four.

A terminal may also be referred to as “UE,” a base station. apparatusmay also be referred to as a “Node B” and a subcarrier may also bereferred to as a “tone.” Moreover, a CP may also be referred to as a“guard interval (GI.)” Furthermore, a cell-specific RS may also bereferred to as “common RS.” Furthermore, a reference signal may also bereferred to as “pilot signal.” Furthermore, a subframe may also bereferred to as “slot.”

Furthermore, an antenna may also be referred to as “antenna port.” Here,a plurality of physical antennas may be used as one antenna port.“Antenna port” refers to a theoretical antenna formed by one or morephysical antennas. That is, “antenna port” does not necessarily refer toone physical antenna, but may refer to an array antenna and so forthcomposed of a plurality of antennas. For example, 3GPP-LTE does notdefine how many physical antennas constitute an antenna port but definesan antenna port as a minimum unit to allow a base station to transmitdifferent reference signals. In addition, an antenna port may be definedas a minimum unit for multiplying a preceding vector as weighting. Forexample, in a base station having eight physical antennas (physicalantennas 0 to 7), physical antennas 0 and 4 transmit R0 with weighting(e.g. weighting factor (1, −1)) and transmit R4 with weightingorthogonal to the weighting of R0 (e.g. weighting factor (1, −1)).Likewise, physical antennas 1 and 5 transmit R1 with weighting (e.g.weighting factor (1, 1)) and transmit R5 with weighting orthogonal tothe weighting of R1 (e.g. weighting factor (1, −1)). In addition,physical antennas 2 and 6 transmit R2 with weighting (e.g. weightingfactor (1, 1)) and transmit R6 with weighting orthogonal to theweighting of R2 (e.g. weighting factor (1, −1)). Moreover, physicalantennas 3 and 7 transmit R3 with weighting (e.g. weighting factor (1,1)) and transmit R7 with weighting orthogonal to the weighting of R3(e.g. weighting factor (1, −1)). By this means, LTE+ terminals canperform channel estimation by demultiplexing respective channels fromphysical antennas 0 and 4 to these LTE+ terminals using R0 and R4.Likewise, LTE+ terminals can perform channel estimation bydemultiplexing respective channels from physical antennas 1 and 5 tothese terminals using R1 and R5, perform channel estimation bydemultiplexing respective channels from physical antennas 2 and 6 tothese LTE+ terminals using R2 and R6 and perform channel estimation bydemultiplexing respective channels from physical antennas 3 and 7 tothese LTE+ terminals using R3 and R7. That is, a base station transmitstwo cell-specific RSs with weighting orthogonal to one another, from twophysical antennas. Even if this RS transmission method is employed, thepresent invention can provide the same advantage as in theabove-described embodiments,

In addition, with the above-described embodiments, although the caseshave been described where LTE+ terminals employs high-order MIMO (MIMOwith eight antennas), the present invention is not limited to this butis applicable to a case in which the receiving side (LTE+ terminals)receives reference signals for more antennas than in 3GPP-LTE, forexample, receives reference signals from a plurality of base stations.For example, although one base station has eight antennas in theabove-described embodiment, the present invention is applicable to acase in which a plurality of base stations have eight antennas inaddition, with the above-described embodiments, assume that 3GPP-LTEuses four antennas, a case has been described as an example wherehigh-order MIMO uses eight antennas by adding four antennas with respectto the case of 3GGP-LTE However, the present invention is not limited tothis, and assume that 3GPP-LTE uses two antennas, high-order MIMO mayuse a total of four antennas by adding two antennas with respect to thecase of 3GPP-LTE. Otherwise, the above-described numbers of antennas maybe combined, and assume that 3GPP-LTE uses two antennas or fourantennas, high-order MIMO may use the number of antennas by adding twoantennas or four antennas with respect to the case of 3GPP-LTE.Otherwise, assume that 3GPP-LTE uses two antennas, high-order MIMO mayuse a total of eight antennas by adding six antennas with respect to thecase of 3GPP-LTE.

In addition, when the concept of antenna port is employed, even if thenumber of actual physical antennas is eight, four antenna ports may bedefined for cell-specific RSs supporting 3GPP-LTE (cell-specific RSsused in both LTE terminals and LTE+ terminals) and other eight antennaports may be defined-for cell-specific RSs supporting high-order MIMO(cell-specific RSs used only in LTE+ terminals). In this case, a basestation can operate such that it transmits cell-specific RSs supporting3GPP-LTE with weighting by two physical antennas per antenna port andtransmits cell-specific RSs supporting high-order MIMO withoutweighting, from each antenna.

In addition, cell-specific RSs may be defined as RSs used to demodulatebroadcast information (PBCH) or PDCCH in its cell, and terminal specificRSs may be defined as RSs used to demodulate transmission data toterminals.

In addition, methods of transforming between the frequency domain andthe time domain are not limited to IFFT and FFT.

Moreover, the present invention is applicable to not only base stationsand terminals, but also all radio communication apparatuses.

Also, although cases have been described with the above embodiment asexamples where the present invention is configured by hardware, thepresent invention can also be realized by software.

Each function block employed in the description of each of theaforementioned embodiments may typically be implemented as an LSIconstituted by an integrated circuit. These may be individual chips orpartially or totally contained on a single chip. “LSI” is adopted herebut this may also be referred to as “IC,” “system LSI,” “super LSI,” or“ultra LSI” depending on differing extents of integration.

Further, the method of circuit integration is not limited to LSI's, andimplementation using dedicated circuitry or general purpose processorsis also possible. After LSI manufacture, utilization of a programmableFPGA (Field Programmable Gate Array) or a reconfigurable processor whereconnections and settings of circuit cells within an LSI can bereconfigured is also possible.

Further, if integrated circuit technology comes out to replace as aresult of the advancement of semiconductor technology or a derivativeother technology, it is naturally also possible to carry out functionblock integration using this technology. Application of biotechnology isalso possible.

The disclosure of Japanese Patent Application No. 2008-163033, filed onJun. 23, 2008, including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a mobile communication system andso forth.

1. A reference signal allocation method comprising: allocating firstreference signals to all resource blocks in one frame, the firstreference signals being used in first radio communication terminalapparatuses supporting a radio communication base station apparatushaving N antennas and also used in second radio communication terminalapparatuses supporting a radio communication base station apparatushaving more antennas than the N antennas; and allocating secondreference signals used only in the second radio communication terminalapparatuses to part of the resource blocks in the one frame.
 2. Thereference signal allocation method according to claim 1, wherein thefirst radio communication terminal apparatuses are LTE terminals, andthe second radio communication terminal apparatuses are LTE-advancedterminals.
 3. The reference signal allocation method according to claim1, wherein the first radio communication terminal apparatuses are radiocommunication terminal apparatuses that operate only in an earlierversion of a communication system than a communication system in whichthe second communication terminal apparatuses operate.
 4. The referencesignal allocation method according to claim 1, wherein the secondreference signals are allocated to the part of the resource blocks indifferent frequency bands between neighboring subframes in the oneframe.
 5. The reference signal allocation method according to claim 1,wherein the second reference signals are allocated to the part of theresource blocks by shifting one or more resource blocks in a frequencydomain on a per subframe basis in the one frame.
 6. The reference signalallocation method according to claim 1, wherein the second referencesignals are allocated to a plurality of the part of the resource blocksin a same subframe in the one frame,
 7. The reference signal allocationmethod according to claim 6, wherein the second reference signals areallocated to the plurality of the part of the resource blocks notcontinuing in a frequency domain in the same subframe in the one frame.8. The reference signal allocation method according to claim 1 whereinthe second reference signals are allocated to the part of the resourceblocks at predetermined subframe intervals in the one frame,
 9. Thereference signal allocation method according to claim 1, wherein anumber of the second reference signals per resource block is smallerthan a number of the first reference signals per resource block.
 10. Thereference signal allocation method according to claim 1, wherein atransmission power of the second reference signals is lower than atransmission power of the first reference signals.
 11. The referencesignal allocation method according to claim 1, wherein the secondreference signals are allocated to the part of the resource blocksvarying in a time domain and a frequency domain on a per cell basis. 12.The reference signal allocation method according to claim 1, wherein anumber of resource blocks to assign the second reference signals arechanged according to a number of the first radio communication terminalapparatuses in a cell.
 13. The reference signal allocation methodaccording to claim 1, wherein subframe intervals to allocate the secondreference signals are changed according to moving speeds of the secondradio communication terminal apparatus,
 14. A radio communication basestation apparatus that transmits first reference signals and secondreference signals, the first reference signals being used in first radiocommunication terminals supporting another radio communication basestation apparatus having N antennas and also used in second radiocommunication terminal apparatuses supporting the radio communicationbase station apparatus having more antennas than the N antennas, and thesecond reference signals being used only in the second radiocommunication terminal apparatuses, the radio communication base stationapparatus comprising: a setting section that sets resource blocks toallocate the second reference signals on a per subframe basis, based onallocation patterns of the second reference signals; and an allocationsection that allocates the first reference signals to all the resourceblocks in one frame and allocates the second reference signals to partof the resource blocks set in the one frame.
 15. The radio communicationbase station apparatus according to claim 14, further comprising abroadcasting section that broadcasts the allocation patterns to thesecond radio communication terminals using broadcast channel signals.